The purification of liquids by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture. Well-known processes include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration, ion exchange, and electrodeionization. Modern electrodeionization units are described, for example, in commonly-owned U.S. Pat. Nos. 5,308,466, issued May 3, 1994, and 5,316,637, issued May 31, 1994, both to Ganzi et al.
One problem associated with many liquid purification devices is the formation of scale on various liquid-contacting surfaces. In the case of electrodeionization apparatus, for example, scale is known to form on ion-exchange membranes which define ion-depleting and ion-concentrating compartments as well as on ion-exchange resins contained within at least some of the compartments.
Numerous methods to prevent or remove scale formation in electrical water purification apparatus have been attempted. For example, U.S. Pat. No. 2,854,394 to Kollsman describes the use of polarity reversal in electrodialysis devices as a way to reduce clogging of pores contained in the membranes used in such devices.
Polarity reversal techniques have been used in electrodeionization devices as well. For example, U.S. Pat. No. 4,956,071 to Giuffrida et al., describes a polarity reversal electrodeionization device that exploits the partial hydrogen and hydroxide regeneration of the resin bed, and provides for non-simultaneous switching of the polarity across the device and the interchange of the concentrating and depleting streams. Continuous production of purified water results. U.S. Pat. No. 5,026,465 to Katz et al., describes a polarity reversal electrodeionization device in which the concentrating and depleting streams are interchanged while, simultaneously, the polarity across the device is switched.
Although the use of polarity reversal for the removal of foulants, contaminants, and scale from the electrodes, membranes, and resins contained within various electrically driven deionization and fractionation devices is known, the results to date have been less than satisfactory. For example, the "time-to-quality", that is, the length of time following polarity reversal required for the new ion-depleting compartments to produce water of acceptable quality, is substantially longer than desired for many applications. Additionally, carbon dioxide gas is typically generated in fluid in ion-concentrating compartments of electrodeionization apparatus. The presence of carbon dioxide in the gaseous state in ion-concentrating compartments causes a disadvantageous increase in electrical resistance across these compartments. Carbon dioxide dissolved in fluid in ion-concentrating compartments exists in equilibrium with carbonic acid, the presence of which is disadvantageous as carbonic acid is weakly ionized, therefore not a good conductor, and not easily removed from solution. Also, chemical and biological control within ion-concentrating compartments in many known electrodeionization devices is lacking.
Another complication associated with the operation of electrodeionization apparatus is that such devices often require further the elimination of foulant particles from process streams that tend to settle between the resin particles within the device, especially when low-quality feed water is used. Such foulant particles tend to plug the device, causing a reduction in the flow of product and concentrate streams and an increase in the pressure drop across the device. The problem of resin fouling is particular to electrodeionization and not to electrodialysis, as electrodialysis compartments are free of ion exchange resins. One option for avoiding this problem is backwashing the apparatus with the resin in place. Such a process is described in U.S. Pat. No. 4,692,745 to Giuffrida et al. Backwashing is an intermittent process where the flow through the concentrating and depleting compartments is reversed during a period in which no power is provided to the device electrodes. The reverse flow effluent comprises a waste stream containing a particular material which is normally discarded to drain. Another option is backwashing the apparatus with the resin removed. Such a process is described in U.S. Pat. No. 5,120,416 to Parsi et al. This process necessarily involves refilling the compartments which contain resin, which is cumbersome and time-consuming. Both of these options necessarily involve apparatus downtime.
Despite the attempts described above, a need exists for an electrodeionization apparatus which avoids the build-up of scale and fouling (or which allows scale and foulants to be removed) while providing a continuous, high purity product stream.
A need also exists for electrodeionization apparatus capable of continuous high-purity product recovery that can be easily fabricated, and that can be manually or automatically sequenced through a polarity reversal process with good time-to-quality.
A need also exists for electrodeionization apparatus allowing good control of chemical and/or biological species in an ion-concentrating compartment array.
Additionally, a need exists for efficient removal of gas generated in ion-concentrating compartments during electrodeionization apparatus operation.